The present invention relates generally to optical systems and, more particularly, to an apparatus for providing selectable optical path lengths for use as an optical time delay unit in an optical phased array beam-steering system.
Microwave radar systems have been used for many years in the field of target detection for purposes of obstacle avoidance, terrain mapping and intrusion detection. More recently, radar systems at infrared wavelengths have taken advantage of the substantially shorter wavelengths of light to provide high resolution definition of target surfaces for highly accurate mappings and object identification. In order to utilize the established technology of the microwave radar systems in optical systems, it has been necessary to develop analogous subsystems.
In recent years, microwave radar antenna systems have relied increasingly on electronic steering, employing phased arrays for rapid beam movement, in contrast to the slow movement afforded by mechanically-steered rotating antennas. Phased arrays rely for their beam directionality on varying the time delay from the source of a common signal to each radiating element of the array. The delay of each element is electronically tuned so that the wavefront radiated toward an off-boresight target simulates in phase, amplitude and direction the parallel rays which would be generated along the boresight axis. Diode phase shift networks are frequently employed in conjunction with each element to provide the necessary time delays and thereby control the radiated beam direction and pulse profile.
Presently available technologies are not sufficiently advanced to supply the need for rapid, large-angle pointing and scanning of optical beams and, in particular, of large diameter, diffraction limited carbon dioxide (CO.sub.2) laser radar beams. In many systems, optical beam steering is currently performed using rotating optical elements. There exists a pressing need for an optical version of the versatile phased array antennas now widely used for microwave radar systems.
A fundamental element in an electrically tunable optical phase shifter is a time delay unit, by which a short burst of infrared light energy, illustratively having a duration of one nanosecond, may be delayed, illustratively by one-fourth nanosecond, so that a composite of delayed and undelayed bursts across the surface of the antenna array is a wavefront which is steered at an angle off boresight. The delay time of such a unit must be electrically selectable in order to effect beam steering.